Generally, a biosensor comprises an electrically insulating base plate, an electrode system including a plurality of electrodes and formed on the electrically insulating base plate using a screen printing method, and an enzyme reaction layer including a hydrophilic polymer, oxidoreductase and an electron acceptor and formed on the electrode system. When a sample liquid containing a substrate is dropped on the enzyme reaction layer of the biosensor, the enzyme reaction layer is dissolved to allow the substrate and enzyme to react with each other. At a result, the substrate is oxidized, and then the electron acceptor is reduced. After such an enzyme reaction finishes, the concentration of the substrate in the sample liquid is determined from an oxidation current obtained by electrochemically oxidizing the reduced electron acceptor.
As a biosensor for quantifying a specific substance contained in a biological sample using an electrochemical manner, a glucose sensor is known. FIGS. 1 and 2 show a structure of the glucose sensor.
FIG. 1 is an exploded perspective view of a conventional biosensor in which a reaction layer is omitted. FIG. 2 is a longitudinal sectional view of the biosensor shown in FIG. 1.
Referring to FIG. 1, silver paste is screen-printed on an electrically insulating base plate 1 to form leads 2 and 3 on a base plate 1. Conductive carbon paste containing a resin binder is then printed on the base plate 1 to form an operating electrode 4 on the base plate 1. The operating electrode 4 is contacted with the lead 2. Electrically insulating paste is then printed on the base plate 1 to form an insulating layer 6. The insulating layer 6 covers all portions except the operating electrode 4 so that the exposed area of the operating electrode 4 is maintained to be constant. Conductive carbon paste containing the resin binder is printed on the base plate 1 to come into contact with the lead 3 and thus to form a ring-shaped counter electrode 5. Subsequently, on or near an electrode system including the operating electrode and the counter electrode, a reaction layer is formed.
The electrically insulating base plate 1 having the reaction layer and a cover 9 having an air hole 11 are bonded to each other via a spacer 10, along dashed dot lines marked in FIG. 1, to manufacture a biosensor. A slit 13 is formed at the spacer 10 to provide a sample supplying path between the base plate and the cover. Referring to a longitudinal sectional view of the biosensor having the above-mentioned structure, a hydrophilic polymer layer 7 is disposed at the electrically insulating base plate 1 having the electrode system, and a reaction layer 8 including enzymes and electron acceptors and a lecithin layer 8a are disposed on the hydrophilic polymer layer 7 in this order.
When a biological sample is contacted with an introduction port 12 of the biosensor having the above-mentioned structure, the biological sample fills the slit 13 acting as a sample receiving space, and at the same time air in the sample receiving space is vented through an air hole 11 formed at the cover 9.
However, since the air hole 11 is formed at the upper part of the biosensor, the biosensor is disadvantageous in terms of its handling due to measurement errors caused by frequent contact with the air hole 11 when using the biosensor. Considering the fact that the reaction progresses immediately after the sample comes into contact with the reaction layer, it is important to rapidly absorb the sample irrespective of viscosity of the sample. However, in the biosensor having the above-mentioned structure, since the air hole 11 for venting air is arranged at the rear side of a sample introduction passage, rapid absorption of the sample is limited. Such limited absorption of the sample causes measurement errors in biosensors that initiate the measurement after checking whether or not the sample is completely introduced.